Duck with permit to be a small bird (5)
I believe the answer is:
owlet
'a small bird' is the definition.
(I've seen this before)
'duck with permit to' is the wordplay.
'duck' becomes 'o'.
'with' becomes 'w' (abbreviation).
'permit to' becomes 'let' (I've seen this before).
'o'+'w'+'let'='OWLET'
'be' acts as a link.
(Other definitions for owlet that I've seen before include "Small hooter" , "who'll give a hoot?" , "Young fly-by-night" , "Young hooter" , "Small bird of prey from Africa and Asia" .)